agent: LiveStream/Row/DetailsRow — Preact live event pane

Ports the row taxonomy in docs/terminal-rendering.md (app.js's
renderStream/renderRichToolUse/renderToolResult/renderTaskEvent) to
real Preact components + hooks:

- lib/streamRow.ts: plain-data StreamRow model (one row = one line/
  panel), lib/classifyEvent.ts: raw stream-json event -> StreamRow[],
  almost entirely dispatching on the backend's precomputed _icon/
  _summary/_category/_body/_body_type fields, same as the old client.
- lib/markdown.ts, lib/linkify.tsx: sanitized-markdown + auto-link
  helpers, same behavior as app.js's mdNode/terminal.js's linkify.
- hooks/useLiveStream.ts: backfill + SSE + seq-dedupe + coalescing,
  reduced to a plain StreamRow[] — deliberately has no opinion on
  scroll position, only on what rows exist.
- components/Row.tsx: renders one StreamRow (flat or details).
- components/LiveStream.tsx: owns the scrollable DOM node + a from-
  scratch sticky-bottom implementation — not the old MutationObserver
  + rAF snap-animation + smoothScrollingUntil gate. stickToBottom is
  plain state driven by the scroll handler; snapping is an instant
  scrollTop write in a useLayoutEffect that runs after Preact has
  already committed the new rows, so it always sees the real
  scrollHeight. A load-older prepend is told apart from a normal
  append via an explicit ref set right before calling loadMore(),
  rather than inferred from DOM mutation shape after the fact.

Wired into Root.tsx below Header/StatusChips; turn_start/turn_end
also nudge useAgentState's refresh() for a snappier badge update than
the plain poll interval.

Reuses @hive/shared/terminal.css's existing row-kind classes as-is —
the taxonomy's visual language isn't what changed, the component
model underneath it is.

frontend/packages/agent/src/lib/markdown.ts

@@ -0,0 +1,25 @@
+// Markdown → sanitized HTML, ported from app.js's `mdNode`. Message
+// bodies rendered into the live stream (assistant text, send/ask/answer/
+// recv payloads) are untrusted (peer-agent / matrix-relayed content,
+// agent-authored files) — `marked` itself no longer sanitizes (v5+
+// dropped the built-in sanitizer), so every parse is run through
+// DOMPurify before it's ever handed to `dangerouslySetInnerHTML`.
+import { marked } from 'marked';
+import DOMPurify from 'dompurify';
+
+marked.setOptions({ breaks: true, gfm: true });
+
+const ESCAPE_RE = /[&<>"]/g;
+const ESCAPE_MAP: Record<string, string> = { '&': '&amp;', '<': '&lt;', '>': '&gt;', '"': '&quot;' };
+
+/** Render `text` as sanitized markdown HTML. Falls back to escaped plain
+ * text if `marked` throws (mirrors app.js's try/catch fallback). */
+export function renderMarkdown(text: string | null | undefined): string {
+  const src = String(text ?? '');
+  try {
+    return DOMPurify.sanitize(marked.parse(src) as string);
+  } catch (err) {
+    console.warn('marked failed', err);
+    return src.replace(ESCAPE_RE, (c) => ESCAPE_MAP[c] ?? c);
+  }
+}